The present invention relates to an input protection device for an electronic device, and more particularly to an input protection device for protecting an electronic device used in an automobile from destructive breakdown and/or malfunction owing to electromagnetic interference and/or high frequency surge.
Electronic devices mounted on an automobile are subject to harsh operating environments. Thus, an input protection device is needed to prevent breakdown and/or malfunction of the electronic devices. Demands on such an input protection device are:
(1) Suppression of high frequency surge, of the order of several hundread voltages, induced by induction load in order to protect the electronic device from destructive breakdown; and
(2) Elimination of electromagnetic interference noise and/or high frequency noise pulse owing to the above-mentioned surge in order to protect the electronic device from malfunction.
U.S. Pat. No. 5,014,155 issued to Abe et al. on May 7, 1991 discloses an input protection device in FIGS. 1, 2 and 3. This known input protection device is adapted to be connected between a switch and an electronic device and circuited between a power supply and ground. The switch is grounded. The input protection device has a current switch, and a circuit. The circuit includes an input terminal for externally connecting with the opposite end of the current limiting resistor and an output terminal for external connection with the electronic device. The circuit includes a power supply diode having an anode connected to the input terminal and a cathode connected to the power supply, a ground diode having an anode connected to the ground and a cathode connected to the input terminal, a bias or pull-up resistor having one end connected to the input terminal and an opposite end connected to the power supply, a capacitor having one side connected to the input terminal and an opposite side connected to the ground, and circuit elements coupling the input terminal with the output terminal. The circuit elements include a comparator having a signal input connected to the one side of the capacitor and a reference input. The elements of the circuit are integrated and formed on a semiconductor substrate. When a high frequency surge is applied to the device and superimposed on an output of the switch, this surge is clamped owing to the power supply and ground diodes and then damped by a filter composed of the current limiting resistor and the capacitor. In order to improve the performance of this filter, it is conceivable to increase the capacitance of the capacitor, since the resistance of the current limiting resistor is limited to such a low level as to allow an electric current above a predetermined value (on order of 1 to 2 mA) to pass through the input terminal, in order to insure conduction through oxidized film formed on the external connection of the circuit to the current limiting resistor.
Assuming that a sufficiently high degree of capacitance is formed in the semiconductor substrate by an ordinary MOS capacitor, it is necessary to provide an input resistor to protect the MOS capacitor from electrostatic destructive breakdown. The provision of the input resistor induces direct-current (DC) regeneration upon application of high frequency surge. This DC regeneration causes a shift in input voltage level. Thus, there may occur a malfunction wherein the output of the comparator switches from a high level to a low level upon application thereto of a high frequency surge when the switch is open, since a voltage applied to the signal input of the comparator may drop below the reference voltage owing to the above-mentioned DC regeneration.
Forming a clamping circuit with the power supply diode and ground diode poses a problem in that, since it is difficult to maintain either a uniform junction capacitance or a uniform capacitance versus voltage characteristic, product-to-product variability is ineligible.
An object of the present invention is to provide an input protection device which, with a reduced number of component parts, provides a capacitance sufficiently great for damping electromagnetic interference noise without causing occurrence of malfunction upon application thereto of high frequency surge.